DSA interview question

All Root-to-Leaf Paths

Medium · Data structures · Topics: tree, dfs, backtracking

The problem

A decision-tree debugger lists every path from the root to a leaf, like "1->2->5", so analysts can see each complete chain of decisions. A DFS carries the path built so far. At a leaf it records the finished path; otherwise it extends the path with the child and recurses. Passing the path by value (or appending and then undoing) keeps sibling branches from seeing each other's nodes.

Example: [1, 2, 3, null, 5] → ["1->2->5", "1->3"].

Function signature

vector<string> binaryTreePaths(TreeNode* root);  // leaf paths from left to right

Test cases

Input (level order)	Output	Explanation
`[1, 2, 3, null, 5]`	`["1->2->5", "1->3"]`
`[1, 2, 3, 4, 5, 6, 7]`	`["1->2->4", "1->2->5", "1->3->6", "1->3->7"]`
`[1, null, 2, null, 3]`	`["1->2->3"]`	One leaf
`[1]`	`["1"]`	The root is a leaf
`[]`	`[]`

(Confirm: a leaf has no children; paths are listed in left-to-right order of their leaves.)

How interviewers phrase it

"Given the root of a binary tree, return all root-to-leaf paths in any order" (LeetCode 257; GeeksforGeeks "Root to leaf paths").

Clarifying questions to ask

What output format? (Strings like "1->2->5", or lists of values.)
Does a single node count as a path? (Yes.)
Can values be negative? (Yes, which only matters for formatting.)

Follow-ups to expect

"Why is it important to check for a leaf, not null?"
"Pass by value or backtrack?"
"How many paths can there be?"
"Related problems?"
"Complexity?"

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